At Site 1131, in addition to routine monitoring of hydrocarbon and H2S gases for safety, analyses were conducted for inorganic carbon, total carbon, nitrogen, and sulfur. The analytical procedures are described in "Organic Geochemistry" in the "Explanatory Notes" chapter.
Concentrations of volatile hydrocarbons and H2S were routinely determined in samples from Hole 1131A using standard Ocean Drilling Program (ODP) headspace or vacutainer sampling procedures. The striking results from Site 1131, as at Site 1127, are the high concentrations of methane (C1) and H2S in part of the section (Tables T5, T6). Four gas pockets occurred from 344.8 to 388.4 mbsf. These were sampled directly through the core liner using a gas-tight syringe (vacutainer). Methane concentrations in these gas pockets ranged from 61,093 to 81,267 ppm (Table T5). By comparison, gas pockets at Site 1127 were more abundant, and methane concentrations in the gas pockets at Site 1127 were about seven times greater, ranging from 329,000 to 585,000 (see "Organic Geochemistry" in the "Site 1127" chapter).
Methane occurs at lower concentrations in the headspace samples from Hole 1131A; the lowest values are at the top and bottom of the sedimentary section, and the maximum values lie at ~200 mbsf (Table T6; Fig. F12). From 1.5 to 55.4 mbsf, methane values are <250 ppm. From 64.9 to 211.1 mbsf, methane increases to 11,458 ppm, the highest value, and then decreases steadily to 45 ppm at the bottom of the section. The C1 headspace profile with depth tracks Corg values in the upper part of Hole 1131A. Low C1 concentrations in the upper 100 mbsf and higher values from 100 to 200 mbsf correlate to a similar Corg pattern in the same depth intervals (see "Inorganic and Organic Carbon, Sulfur, and Nitrogen"). In contrast to the bimodal distribution of C1 at Site 1127 (see "Organic Geochemistry" in the "Site 1127" chapter), methane has a unimodal distribution in Hole 1131A.
Ethane (C2) was present from 26.9 mbsf to the base of Hole 1131A at concentrations as high as 25 ppm in headspace samples; most values are between 10 and 20 ppm (Table T6). The C1/C2 values in headspace samples from the same depth interval first increase from 40 to 476 and then decrease to a low of 76 at the base of the section (Table T6; Fig. F12). Although the initial increase and subsequent decrease in the C1/C2 ratio is atypical for sedimentary basins, and the values are in the range indicative of thermogenic hydrocarbon sources based on the ODP "Guidelines for Pollution Prevention and Safety" (JOIDES PPSP, 1992), the change in ratio with depth mirrors the increase and decrease in methane concentration. That is, C2 concentrations do not vary significantly from 72.1 mbsf to the base of Hole 1131A; the change in C1/C2 results entirely from variations in C1. Heavier hydrocarbon gases (C3+) were detected in a few headspace samples from 359.1 to 530.6 mbsf, as well as in the four gas pockets.
The concentrations of H2S in the four gas pockets in Hole 1131A range from 107,651 to 156,608 ppm, exceeding the highest value measured at Site 1127 and possibly any other ODP site (Table T5). The H2S in headspace samples from Hole 1131A reaches concentrations as high as 59,681 ppm and shows a bimodal distribution with maxima at 136.4 and 359.1 mbsf. The headspace concentrations are lower than those found in gas pockets, although, as with the gas pockets, the H2S values are higher in Hole 1131A than at Site 1127 (Table T6; see "Organic Geochemistry" in the "Site 1127" chapter).
Throughout Hole 1131A, carbonate values are uniform and range primarily from 86 to 94 wt% (Table T7; Fig. F13).
Organic carbon (Corg) reaches values as high as 1.0 wt% (Table T7; Fig. F13). From the surface to a depth of ~125 mbsf, Corg values are primarily <0.4 wt%. In the interval from 125 mbsf to total depth, Corg values increase to between 0.4 and 1.0 wt%. Methane concentration in the interval from 125 to ~300 mbsf is also relatively high, apparently related to the higher Corg content.
Nitrogen concentrations are all <0.1 wt%, and many samples have no detectable nitrogen. Sulfur was detected in only three samples (Table T7).